1. Field of the Invention
The present invention relates to a power supply device for a vehicle, and more particularly, to a power supply device for a vehicle to be mounted in an idling-stop vehicle which temporarily stops the engine automatically when the vehicle comes to a stop, such as at a traffic light.
2. Description of the Related Art
In recent years, idling-stop vehicles that temporarily stop the engine automatically when the vehicles come to a stop, such as at a traffic light, have been used to reduce fuel consumption and emissions.
In a vehicle of this type, the engine is automatically stopped based on predetermined idling determination information such as the vehicle speed or the degree of opening of the accelerator when the vehicle is presumed to have stopped, and then automatically started by the starter in order to prepare the vehicle to start moving when an engine start condition indicating that the driver wants to start moving is satisfied.
In general, since the starter for starting an engine consumes a large amount of power, a phenomenon of a temporary drop in the voltage of a battery output system at the moment of starting the engine may occur.
Such a phenomenon becomes significant especially when a battery used as the power source is heavily discharged due to street driving in which stopping and starting of movement are frequently repeated.
When such a voltage drop occurs, problems, such as resetting of microcomputers used in electrical equipment in the vehicle, thereby losing memory contents, such as contents that have been learned up to that time, or temporary dimming of the instrument lighting or other lighting, thereby severely impairing the quality feeling of the vehicle, may occur.
Therefore, a configuration has been disclosed in which a boosting circuit for voltage compensation is provided in an idling-stop vehicle in order to prevent a voltage drop in the battery output system during restarting of the engine, and the boosting circuit is operated during a period in which the starter is operating upon starting of the engine (see, for example, Japanese Unexamined Patent Application Publication No. 2005-237149).
In addition, since a battery can be replaced by the owner, there may be a possibility of the polarities thereof being erroneously connected. Therefore, a protection circuit that prevents current from flowing when a reverse connection is made has been disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2-197441.
A method for forming a protection circuit that prevents current from flowing when the polarities of a battery are mistakenly connected in reverse in a circuit that compensates, by using a boosting converter, for an output voltage drop due to a large power consumption of the starter during restarting of the engine as described above will be explained with reference to FIGS. 7 and 8.
FIG. 7 illustrates a configuration including a battery 1 provided as a DC power source, a first load 6, a first capacitor 2, a boosting converter circuit 3, a control circuit 4 which controls the boosting converter circuit 3 and a connector 5 to which the output voltage of the boosting converter circuit 3 is supplied, wherein the connecter 5 is connected to a second load (not shown).
The first load 6 includes a starter 61, an engine 62, and a generator 63. The starter 61 is supplied with DC voltage from the battery 1 during starting of the engine in order to start the engine 62. When the engine starts, power is generated by the generator 63 by using rotation as a power source in order to charge the battery 1.
The boosting converter circuit 3 includes an inductor 31, a first semiconductor switching element 32 defined by a field-effect transistor or other suitable semiconductor element, a first diode 34 and a second capacitor 35. The inductor 31 and the first semiconductor switching element 32 are connected in series to the ends of the battery 1. The anode of the first diode 34 is connected to the connection point between the inductor 31 and the first semiconductor switching element 32, and the cathode of the first diode 34 is connected to an output terminal of the connector 5. The second capacitor 35 is connected to the ends of the connector 5.
With automobiles, it is often the case that the battery 1 can be replaced by the user. Therefore, it is necessary to provide, in the power supply device for a vehicle, a protection circuit that prevents current from flowing in the reverse direction when the user mistakenly connects the polarities of the battery 1 in reverse. In order to provide such a function, a relay switch 8 shown in FIG. 7 or a third diode 9 shown in FIG. 8, for example, may be used to enable conduction only when a forward current flows. However, there is a problem in that when the output current of the power supply device for a vehicle is extremely large, the forward power loss of the third diode 9 or the power consumption of the relay switch 8 becomes too large to ignore, which causes the battery life to be shortened.
In addition, since the vehicle weight is large for large vehicles that are equipped with power steering devices, a high power is required for a motor used in the power steering device, and therefore the size of the motor must be increased. In order to prevent such an increase, a voltage greater than the output voltage (12 V) of an existing lead acid battery may be required. However, even in this case, a large current flows, and the loss cannot be ignored in an existing power supply device using a diode or a relay switch.